This invention relates to a commutation circuit for electrically connecting and disconnecting a load to and from a source of direct current and more particularly to a forced commutation circuit for a semiconductor controlled rectifier such as a uni-directional triode thyristor.
In order to turn off unidirectional triode thyristors (which are abbreviated hereinafter to "thyristors") in its conducting state, it is widely practiced to use a commutation capacitor, the discharging current from which reversely biases the thyristor. For example, well known McMurray and Bedford inverters and direct current (DC) choppers called Johnes or Morgan circuits have included a commutation capacitor. At present, it is prevailingly accepted that the commutation capacitor used in such inverters and DC choppers can be most suitably operated with its coefficient of utilization being about 43 percent.
Also there have been already known commutation circuits including a diode (which may be called the "commutation, bypass or flywheel diode") connected in opposite parallel circuit relationship with the thyristor, and a reactor (which may be called the "commutation reactor") connected serially to the commutation capacitor and in parallel to the thyristor. The last-mentioned commutation circuits have only increased in the coefficient of utilization by several percent as compared with the first-mentioned circuits.